1. Field of the Invention
The present invention relates to a display device for vehicles displaying an image outside a windshield glass. More particularly, it relates to a display device for assisting a vehicle driver passing through a narrow road or parking a vehicle closely to a road side.
2. Related Art
Japanese Patent Application Laid-open No. 61-249849 discloses a display device having a display emitting a vehicle width mark (an example of a display image), a lens converging the vehicle width mark as divergent light emitted by the display to parallel light, and a mirror (an example of optical path change device) directing the vehicle width mark passing through the lens to reflect toward a front windshield. Since the vehicle width mark projected on the windshield is reflected in a slightly diffused condition, the vehicle width mark (image) is displayed at an outside position (for example, 10 m ahead) being a specified distance away from the windshield.
However, positions of eye height of drivers are not fixed and the drivers have eye height varying from high to low. Since the image is formed on an extended line connecting the image reflected on the windshield with the eye position, the image moves up and down undesirably due to variations of the eye height positions of the drivers.
In order to prevent the image from moving up and down, a device disclosed in Japanese Utility Model Laid-open No. 2-33733 is known. In this device, a display device having a display and a lens is interlocked with a height adjustable tilt steering wheel. When the tilt steering wheel is held at a high position, a reflection position of a vehicle width mark projected on a windshield from the display device moves upward on the windshield. On the other hand, when the tilt steering wheel is held at a low position, the reflection position of the vehicle width mark projected on the windshield from the display moves downward on the windshield.
A rotational center of the display device is close to the windshield. In fact, when the display device rotates, the display position of the image undesirably moves up and down around the center on a point between the windshield and the image.
Further, in the display device disclosed in the Japanese Patent Application Laid-open No. 61-249849, since the speed of a vehicle is not constant and varies from low to high, drivers more often look relatively nearby when running at low speed. On the other hand, when the speed of the vehicle is high, the drivers look farther away. However, when the display position of the vehicle width mark is always fixed (10 m ahead), displaying the vehicle width mark cannot have enough effect.
In order to overcome this drawback, Japanese Patent Application Laid-open No. 60-183240 teaches another device in which a Fresnel lens converges a display image in a divergent light form emitted by a display to a parallel light form and directs the display image passing the Fresnel lens toward the windshield along an optical path of the display. Thus, by moving a position of the Fresnel lens along the optical path corresponding to the speed of the vehicle, a position where the image projected on the windshield is moved.
However, in this device, since the Fresnel lens moves along the optical path, when the display distance of the image changes, the image moves along the optical path. That is, the image does not move parallel to the surface of the ground or road, instead, the image undesirably moves up and down relative to the ground.